About fire

What is fire and how to control it

What is fire?

We define fire as a chemical reaction which reduces organic substances like cellulose in trees and shrubs to inorganic substances such as carbon dioxide, water and black soot, and in the process emit a large amount of heat.

What factors control a fire?

There are five critical factors in starting and maintaining a fire: a fuel source, oxygen, temperature, wind and humidity.

Oxygen
Oxygen is what ignites and maintains the fire as it gets used in the chemical reaction.

Fuel
A fuel is anything that is capable of igniting or sustaining combustion in the presence of oxygen and heat. For instance, long dry grass, trees with a high oil content in their leaves and bark, and dead leaves are all examples of fuel because of the ease in which they can be ignited.

Temperature
Temperature is the means of bringing closer the fuel to the point of igniting in the presence of oxygen.

Humidity
Humidity helps to either make it harder or easier to ignite the fuel depending on how much water is in the air and/or in the fuel itself.

Wind
Wind will either dry out or add moisture to the fuel as well as increase the amount of oxygen added to the fuel when it is burning.

In essence, the optimum conditions for starting and maintaining a fire outdoors is a prolonged period of dry hot winds moving over dry land on a clear day (where air pressure is high and oxygen levels are at its maximum at ground level) and there is plenty of fuel on the ground to burn.

How do we control a fire?

Fuel, heat, oxygen, humidity and wind all help to ignite and maintain fire. Reduce the fuel, lower the temperature of the fuel, increase the humidity or amount of water in the air and/or fuel, reduce the wind created by the flames as they absorb oxygen, or cutting off the oxygen supply will stop a fire.

Generally, there is little we can do about the oxygen levels in the air other than to minimise wind speed or use other means to stop the flames from absorbing oxygen from the air.

You will sometimes see fire-fighters use blankets to smother a fire. This has the effect of reducing the oxygen reaching the flames and therefore increasing the success rate of putting out the fire.

We can choose lower temperature conditions such as those prevalent at night.

Fighting a fire at night is better than during the day as the relative humidity is higher at night and the temperature is low. This results in a lower intensity fire at night than during the day.

We can choose conditions of high humidity such as those prevalent on cloudy days and in the tropical areas.

Fire-fighters prefer to combat a fire on a cloudy day, not only because the temperature is lower, but also because there is more moisture in the air and on the ground. Even a very hot day will not create fires if the humidity is high. It requires low humidity combined with high temperatures to increase the probability of a fire from happening. This is why it is rare to see major bushfires in the tropics.

We can choose a time when there is little or no wind, such as right in the middle of the night or day, but never inbetween (e.g. the early mornings or evenings), or somehow stop the flames from creating a wind of their own at ground level as it sucks oxygen into the fire.

Fire-fighters unanimously agree it is better to combat fires on a windless day. The presence of any strong, dry wind during a fire is perhaps the single most dangerous and important factor in determining how to control a fire once it has started. Generally, the higher the wind speed and the more variable the wind direction, the more dangerous it is to control a fire. When fire fighting, it is absolutely critical to know the wind speed and direction and to be kept up-to-date on that information at all times.

But the most effective, safest and easiest means of controlling any fire, is to control the amount of fuel available to ignite and sustain a fire.

You will sometimes see numerous strategically placed dirt roads running through forests or people deliberately burning off certain areas in an attempt to reduce fuel levels. Fire-fighters believe this is the most powerful way of managing fires on an environmental scale.